PTERIDOPHYTES 165 
In passing fromMarattiaceae to Polypodiaceae, it is interesting to note 
the changes in the character of the sporangium stalk. Beginning with 
sessile sporangia, there is a transition to very short stalks, and finally 
the very long slender ones that characterize the Polypodiaceae are 
reached. This change in the character of the stalk accompanies the 
reduction in the output of spores, so that perhaps 
among leptosporangiate ferns the development of 
an elongated sporangium stalk is of more impor- 
tance than a large output of spores from a single 
sporangium. 
Sorus. The sorus is a feature of most Filicales, 
being a definitely limited group of sporangia (figs. 
383, 384). Sori vary in form and in arrangement, 
and are useful as taxonomic characters Sporangia 
do not always form sori ; in some cases they are FlG - 39- Mature 
scattered over the surface of the leaf, in other cases s ?^ 
of the Polypodmceae, 
they form in a continuous band along the margin, showing the incom- 
Indusium. The indusium is a flaplike outgrowth P lete vertical annulus 
t ^ f ^L i c t> L j and the transverse 
from the surface of the leaf which protects the de- 
veloping sorus (figs. 369, 373, 383). It is exceed- 
ingly variable in form; in some cases forming a pouch, in others an 
overarching shield (fig. 383), in others a cup, etc. It is sometimes 
lacking entirely, and in certain cases it is replaced by the inrolled 
leaf margin, which is then spoken of as a " false indusium." These 
variations in the indusium are so constant for different groups as to be 
very useful as taxonomic characters. 
Gametophyte. The gametophyte of the Filicineae, better known 
as the prothallium, is a small green thallus, with rhizoids, and resembles 
the gametophyte of very simple liverworts (fig. 391). The most primi- 
tive ferns (Marattiaceae) have the largest and most massive gameto- 
phytes. In general, the development of the gametophyte passes through 
the stages indicated for liverworts (p. 98). In germination there pro- 
trudes from the spore a papillate outgrowth which is cut off by a wall 
as a cell and develops into the filamentous stage, the length of the fila- 
ment being related to available light. Following this stage is that of the 
apical cell with two cutting faces, by means of which the gametophyte 
is broadened. Then follows the group of initials, by means of which the 
gametophyte is matured, in the course of which the usual apical notch 
is developed. The axial region of the gametophyte is much thicker than 
